The present invention relates to oximeter sensors, in particular a fetal pulse oximetry intrauterine sensor.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the clinician.
It is desirable that photoelectric pulse oximetry also be useful for monitoring the blood characteristics and constituents of a fetus. For example, monitoring fetal oxygen levels provides an effective way to detect and provide indications for treating hypoxia in the fetus during labor.
A number of different designs are used for fetal sensors. U.S. Pat. No. 5,247,932 shows a bladder between the fetus and the uterine wall which presses the active face of the sensor against the fetus' skin. U.S. Pat. No. 5,377,675 discloses a sensor using a fulcrum to bias the sensor against the fetus. PCT Published Application No. W091/07910 uses an inflatable sac to wedge the sensor against the fetus.
FIG. 1 illustrates the insertion of a fetal pulse oximeter sensor into a uterus so that a sensor portion 10 is against the head of a fetus 12. The sensor 10, or sensor head, is connected to a sensor rod 14 which extends out of the uterus. It is desirable to have this rod bend to go around the side of the fetus' head. In one type of sensor, the sensor rod is made by an extrusion process, which does not allow a bent shape. The sensor head is attached and sealed with an overmold, which may be shaped to produce a fulcrum as shown in U.S. Pat. No. 5,377,675.
U.S. Pat. No. 5,247,932 is one example of using a stylet to allow insertion of a sensor and then the subsequent bending needed. The sensor rod is made by a process which allows it to be preformed to be bent, and the metal stylet is inserted into a channel in the sensor rod to straighten it for insertion. After being inserted, the metal stylet is removed so that the sensor rod returns to its original bent shape, biasing the sensor against the fetus. Typically, the sensor rod is manufactured to be bent. One way to manufacture this way is to extrude the rod, then insert a permanent stylet which is curved. A separate lumen is provided for the metal stylet which will straighten the sensor for insertion.